The recovery of residual oil remaining in place in a gas cap following primary oil recovery is problematic because capillary forces usually trap a large fraction of the residual oil in the gas cap pores. Water-alternating-gas (WAG) floods have commonly been used in place of conventional water or gas floods for displacing residual immobile oil from formations having already undergone primary recovery. It is believed that the gas flood renders the oil in place more mobile so that it may be readily swept from the formation by a subsequent waterflood. Therefore, it has been suggested to employ a WAG flood in a gas cap to more effectively recover residual oil therefrom. Unfortunately, WAG floods have not proven as effective when applied to a gravity drainage mechanism, such as found in most gas caps, than when applied to a horizontal sweep mechanism.
In a conventional WAG process, as taught for example in U.S. Pat. No. 4,846,276 to Haines, the water is injected into the formation in relatively large volumes approximating the volume of injected gas. Both the injected water and injected gas are designed to horizontally sweep the formation and displace fluids residing therein under favorable mobility conditions. However, when the flood vertically sweeps the formation, as in the case of a gas cap, the water simply drops through the formation under the force of gravity bypassing the oil-containing pores. Even where a preceding gas has increased the mobility of the oil, contacting of the oil with subsequently injected water is generally very poor in a gas cap.
U.S. Pat. No. 3,788,398 to Shephard recognizes the shortcomings of a conventional WAG flood in a gas cap and attempts to remedy the problem by injecting a gas upward from beneath the waterflood to improve horizontal distribution of the water across a gas cap that is undersaturated as to water. Shephard relies on the waterflood to enter the gas cap and displace the gas therefrom. The displaced gas then carries mobile oil with it to the production well.
It has been found, however, that the method of Shephard is ineffective where the gas cap is water saturated. A water saturated gas cap cannot accommodate the quantities of water sufficient to adequately sweep the gas cap in the manner required by Shephard. Thus, the injected water simply fingers through the gas cap in the same manner as a conventional WAG flood.
Accordingly, it is an object of the present invention to provide a process for effectively recovering residual oil from a gas cap following primary recovery. It is another object of the present invention to effectively recover residual oil from a gas cap that is water saturated, and particularly wherein the level of water saturation is relatively low. It is further an object of the present invention to provide a process for sequentially flooding a gas cap with cycles of water and gas, wherein the water portion of each cycle effectively contacts the residual oil trapped by capillary pressure in the gas cap pores.